Developing methods to control the molecular orientation of crystals is a major goal of the material fabrication industry and has occupied the focus of many studies and research efforts. In particular, the total alignment and orientation of particulate and dense crystalline ceramic films at low temperatures is important for a number of technological applications such as chemical and biological sensors, optical wave guides, optical coatings, and films with magnetic and electrical functions (e.g., magnetic information storage). The commercial value of biosensors alone, has been estimated at $150-200 million with predictions for growth to several hundred billion dollars within the next decade (Kramer, J. AOAC Intern. 79: 1245 [1996]). For many of these applications, conventional processing techniques (e.g., high vacuum beam technologies) cannot be used because they require extremely high temperatures or pressures that would destroy delicate components of the devices. Unfortunately, control of total crystal alignment is not achieved by any of the currently available methods of crystal growth and production at mild temperatures and pressures.
However, in nature, the mineralization of highly oriented inorganic crystals occurs routinely at organic templates (e.g., proteins and oligosaccharides). Biosynthesis of these materials evolves in aqueous media, at mild temperature and pressure conditions. Biological organisms are capable of producing crystals with specific size, morphology, orientation, and texture (Lowenstam and Weiner, On Biomineralization, Oxford Univ. Press, New York, [1989]; and Berman et al., Science 259: 776 [1993]). Such inorganic minerals are used by the organisms for support structures, armor, weapon, gravity sensors, magnetic detectors, and homeostatic devices (Heywood, Microsc. Res. Tech. 27: 376 [1994]).
Organisms achieve control over crystal properties with the use of an organic polymeric "matrix" of highly acidic macromolecules. One mechanism used to orient the crystals involves intercalation of the macromolecules into the crystal lattice (Berman et al., supra.). In certain cases, the minerals that are formed by biological organisms are uniquely oriented or co-aligned relative to the organic matrix (Weiner, CRC Crit. Rev. Biochem. 20: 365 [1986]). To date, however, the in vitro synthesis of novel organic-inorganic composites, with properties analogous to those produced in vivo, have not been achieved and continue to challenge materials scientists.
The nature of the organic-inorganic interactions during crystal formation at biological matrices is poorly characterized. Studies have indicated that the stereochemical match between the organic and inorganic interfaces is a predominant factor in determining the specific nucleation face type generated. However, there are several difficulties that have prevented accurate analyses of these phenomena. To date, structural information regarding the organic monolayer templates have only been obtained in the absence of mineralization (e.g., from grazing incidence x-ray diffraction, electron diffraction, and x-ray reflectivity [Majewski et al., Adv. Mater. 26: 7 (1995); and Jacquemain et al., Angew. Chem. Int. Ed. Engl. 31: 130 (1992)]). Additionally, it is believed that synergistic changes in the organic template structure occur upon interaction with solid interfaces, as is the case when crystals form at the hydrophilic head-group regions of membranes (Heywood et al., J. Chem. Soc. Faraday Trans. Part 2, 87: 735 [1991]). In such cases, in situ structural information regarding the organic template is even more elusive, as dynamic changes in monolayer organization may occur. These difficulties have, in part, prevented the development of techniques for controlling the alignment of crystal growth at biological matrices.
Simplified surfactant molecular assemblies (Ulman, An Introduction to Ultrathin Organic Films, Academic Press, New York [1991]; and Fendler, Membrane Mimetic Chemistry, Wiley, N.Y. [1982]) resembling biological membranes (e.g., monolayers, multilayers, or vesicles) have been used in attempts at oriented crystalline growth. Several groups have demonstrated the nucleation and growth of organic (Landau et al., Nature 318: 353 [1985]) and inorganic crystals at these monolayer assemblies (Landau et al., Mol. Cryst. Liq. Cryst. 134: 323 [1986]; Heywood and Mann, Adv. Mater. 6: 9 [1994]; and Zhao et al., J. Phys. Chem. 96: 9933 [1992]). However, the crystals produced by these methods are randomly oriented or oriented only in the direction normal to the membrane plane (i.e., oriented in a manner where a given face of a crystal is in contact with the plane of the template upon which it is grown) (Heywood and Mann supra; and Zhao et al., supra.) and thus are not suitable for many applications.
Current technologies do not provide methods for totally oriented crystal growth (i.e., orientation along an identifiable structural feature of the template with crystals oriented about all three crystallographic axes) under mild conditions of temperature and pressure. In these prior attempts, the crystal axes in the plane of nucleation have not been aligned with a structural parameter of the nucleation surface (i.e., do not feature totally oriented crystal growth). Additionally, the current methods of producing crystalline ceramic films are not energy efficient and are not compatible with the integration of these films with other materials (e.g., plastics and polymers). Therefore, the art is in need of a method that produces crystals with controlled alignment, in an inexpensive and energy-efficient manner, and that can be generated and used under mild pressures and temperatures.